Nitride semiconductors represented by, e.g., gallium nitride (GaN) have advantages, such as a high breakdown voltage, a high electron saturation velocity, and high electron mobility due to their wide band gaps, and another advantage, such as a high electron density in a heterojunction. Thus, nitride semiconductors have been studied and developed in order to be used for, e.g., high-breakdown-voltage power electronic devices, or high-speed devices for the millimeter wave band.
In particular, heterojunction structures in which nitride semiconductor layers having different band gaps are laminated together, or quantum well structures or superlattice structures in which a plurality of such heterojunction structures are laminated together are capable of controlling the degree of modulation of the electron density in a device, and thus, have been utilized as principal structures of devices using nitride semiconductors.
Examples of semiconductor devices having a heterojunction structure and using nitride semiconductors include a heterojunction field-effect transistor (HFET) (see, for example, Japanese Patent Publication No. 2002-16245 (Document 1)).
The HFET includes, for example, an operation layer formed on a substrate, and made of GaN, a barrier layer made of undoped aluminum gallium nitride (AlGaN), and source, drain, and gate electrodes formed on the barrier layer.
AlGaN has a larger band gap than GaN. Thus, electrons resulting from the difference between the amount of spontaneous polarization of AlGaN and that of GaN and the difference between the amount of piezoelectric polarization of AlGaN and that of GaN, electrons resulting from n-type impurities with which the barrier layer is doped as required, electrons resulting from other uncontrollable defects in the semiconductor layers, and other electrons are densely accumulated at the heterojunction interface between the operation layer and the barrier layer to form a two-dimensional electron gas (2DEG) layer thereat. Electrons in the 2DEG layer function as channel carriers through a field-effect transistor.
When a cathode (ohmic) electrode and an anode electrode are each formed on a structure of nitride semiconductor layers laminated together to form a heterojunction interface, this provides a Schottky barrier diode (SBD) in which electrons in a 2DEG layer function as channel carriers through the diode (see, for example, Japanese Patent Publication No. 2004-31896 (Document 2)).
In order to use semiconductor devices using nitride semiconductors as high-breakdown-voltage power electronic devices, or high-speed devices for the millimeter wave band, the on-resistance needs to be reduced. Principal factors responsible for the on-resistance include the sheet resistance of a channel layer and the contact resistance between an electrode and a semiconductor layer.
For example, in typical HFETs and SBDs, the contact resistance is increased for the following reason. Specifically, source/drain electrodes and a cathode electrode are formed on an undoped AlGaN layer. In this case, electrons must travel beyond the potential barrier of the undoped AlGaN layer, and reach a 2DEG layer. Consequently, the contact resistance is increased.
In order to reduce the contact resistance, for example, the distance between each of the electrodes and the 2DEG layer is selectively reduced. In order to selectively reduce the distance, a portion of an AlGaN barrier layer or the entire AlGaN barrier film is removed to form a contact portion having an inclined bottom or side surface and having a recessed cross section, and an ohmic electrode is formed on the contact portion (see, for example, Japanese Patent Publication No. 2005-129696 (Document 3), and Japanese Patent Publication No. 2007-053185 (Document 4)).
In the structure of Document 3, an AlGaN layer includes an inclined contact portion. With this structure, the electron gas concentration in a portion of the 2DEG layer immediately below the contact portion and the distance from the bottom of the contact portion to the 2DEG layer, which have conventionally been in a trade-off relationship, can each be any value. Consequently, ohmic contact can be provided in a region of the AlGaN layer where the contact resistance and the electron gas concentration are optimized.
In the structure of Document 4, a recess is formed to pass through an AlGaN layer, cross the heterojunction interface, and have a side surface which is inclined at the depth of the heterojunction interface. With this structure, not only ohmic contact through the AlGaN layer between each of the electrodes and the 2DEG layer, but also direct contact therebetween on the side surface of the recess can be provided to reduce the contact resistance.